[lldb] make it easier to find LLDB's python

It is surprisingly difficult to write a simple python script that
can reliably `import lldb` without failing, or crashing.   I'm
currently resorting to convolutions like this:

    def find_lldb(may_reexec=False):
		if prefix := os.environ.get('LLDB_PYTHON_PREFIX'):
			if os.path.realpath(prefix) != os.path.realpath(sys.prefix):
				raise Exception("cannot import lldb.\n"
					f"  sys.prefix should be: {prefix}\n"
					f"  but it is: {sys.prefix}")
		else:
			line1, line2 = subprocess.run(
				['lldb', '-x', '-b', '-o', 'script print(sys.prefix)'],
				encoding='utf8', stdout=subprocess.PIPE,
				check=True).stdout.strip().splitlines()
			assert line1.strip() == '(lldb) script print(sys.prefix)'
			prefix = line2.strip()
			os.environ['LLDB_PYTHON_PREFIX'] = prefix

		if sys.prefix != prefix:
			if not may_reexec:
				raise Exception(
					"cannot import lldb.\n" +
					f"  This python, at {sys.prefix}\n"
					f"  does not math LLDB's python at {prefix}")
			os.environ['LLDB_PYTHON_PREFIX'] = prefix
			python_exe = os.path.join(prefix, 'bin', 'python3')
			os.execl(python_exe, python_exe, *sys.argv)

		lldb_path = subprocess.run(['lldb', '-P'],
			check=True, stdout=subprocess.PIPE,
				encoding='utf8').stdout.strip()

		sys.path = [lldb_path] + sys.path

This patch aims to replace all that with:

  #!/usr/bin/env lldb-python
  import lldb
  ...

... by adding the following features:

* new command line option: --print-script-interpreter-info.  This
   prints language-specific information about the script interpreter
   in JSON format.

* new tool (unix only): lldb-python which finds python and exec's it.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D112973

lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp

@@ -998,20 +998,6 @@ bool PythonFile::Check(PyObject *py_obj) {
 #endif
 }
 
-namespace {
-class GIL {
-public:
-  GIL() {
-    m_state = PyGILState_Ensure();
-    assert(!PyErr_Occurred());
-  }
-  ~GIL() { PyGILState_Release(m_state); }
-
-protected:
-  PyGILState_STATE m_state;
-};
-} // namespace
-
 const char *PythonException::toCString() const {
   if (!m_repr_bytes)
     return "unknown exception";